Peaks are observed in a depth dependent power spectrum of P-wave
velocity fluctuations determined from an inversion of P wave coherences
observed by the USArray. These peaks correlate with the depths of the
majority of silicate mineral phase changes predicted by a thermodynamic
model of the upper 1000 km of a pyrolitic mantle. To within ± 25 km we
identify the phase change of orthopyroxene to HP-clinopyroxene at 275
km, the olivine to wadsleyite phase change at 425 km, the wadlsleyite to
ringwoodite phase change at 505 km, and the initiation of an akimotoite
phase at 600 km and a signature of a phase change at 775 km, both
associated with the existence of fragments of subducted oceanic crust.
Non-detection of a phase change at or near 660 is consistent with the
phase change of ringwoodite to Mg-perovskite and magnesiowusite
occurring over a depth interval much smaller than 25 km.